feat(psbt): add PSBT signing with xprv#56

OttoAllmendinger merged 1 commit intomasterBitGo/wasm-miniscript:masterfrom

BTC-1845.add-psbt-signWithXpubBitGo/wasm-miniscript:BTC-1845.add-psbt-signWithXpubCopy head branch name to clipboard